Truck Bolster

A truck bolster is the cast cross member of a three-piece truck. It spans between the two sideframes and is the part on which the car body rests. It is one of the three principal castings, along with the two sideframes, that give the three-piece truck its name.

Where It Sits

The bolster does not bolt to the sideframes. Instead it passes through the spring window in each sideframe and floats on top of the spring groups, one group at each end. This sprung mounting lets the bolster move vertically against the springs to cushion the load and lets the truck warp as it follows curves.

What It Does

At the center of the bolster is the center bowl, a dished bearing surface. The car body center plate sits in this bowl and the whole car body pivots on it, so the bolster carries the full weight of the body and its load and passes that load down through the springs into the sideframes and on to the wheelsets. Near each end the bolster also carries a side bearing surface that works against the body side bearing to control roll and hunting.

Wear and Inspection

The center bowl is protected against wear by a vertical wear liner in the bowl wall and a horizontal wear liner on the bowl floor. Bolster condemnable conditions are covered by the applicable AAR field manual rule.